Field of the Invention
The present invention relates to an electrical discharge machine using a rust inhibitor for inhibiting corrosion of a workpiece, and more particularly, to an electrical discharge machine having a concentration detection function for the rust inhibitor.
Description of the Related Art
A workpiece (e.g., iron material or cemented carbide) is corroded if it is remain immersed in a working fluid (water) in a machining tank for a long period of time as it is machined by an electrical discharge machine. If the workpiece is corroded, it affects the external appearance, precision, strength and the like of a resulting machined product. In general, to prevent the corrosion of the workpiece, a rust inhibitor is added to the working fluid (water).
Active ingredients of the rust inhibitor can be roughly divided into inorganic and organic compounds. Organic amines, carboxylic acids and the like may be given as specific examples of the organic compounds. An organic compound adheres to the surface of the workpiece and forms a protective organic film on the workpiece, thereby achieving an anti-rust effect.
Although the anti-rust effect is reduced if the concentration of the rust inhibitor is too low, various troubles are caused if the concentration is too high (for example, degradation of the pipings of rubber or resinous material may possibly be accelerated). Therefore, the concentration of the rust inhibitor should be kept within an appropriate range.
The rust inhibitor adheres to sludge generated by machining and piping as well as to the workpiece surface and is exhausted, so that its concentration is reduced. When machining is not in progress, moreover, the concentration of the rust inhibitor increases due to evaporation of the working fluid (water). It is important to detect and control the concentration of the rust inhibitor.
Japanese Patent Application Laid-Open No. 2011-20185 discloses a technique that uses ionic (inorganic-compound) rust inhibitors such as sodium hydroxide, potassium hydroxide, ammonium hydroxide and the like for aqueous working fluids for electrical discharge machines (see, Paragraphs [0024] and of 2011-20185 application). The concentrations of these rust inhibitors are obtained based on an electrical conductivity measured by a conductivity meter of a liquidity measuring sensor.
Whereas in a concentration detection method for an organic rust inhibitor (active ingredient: organic compound), the concentration of the organic compound is generally calculated from the UV absorption intensity by means of high-speed chromatography. On an underequipped manufacturing floor, however, it is hard to detect the concentration of the organic compound in water. The technique disclosed in Japanese Patent Application Laid-Open No. 2011-20185, which is adapted to be applied to inorganic-compound rust inhibitors, cannot be applied to the measurement of the concentrations of organic-compound rust inhibitors. In common methods, therefore, the detection of the concentrations of such rust inhibitors in a working fluid is abandoned and a rust inhibitor is added regularly or after rusting is caused. According to any of these methods, however, the concentrations of the rust inhibitors in the working fluid cannot be confined within an appropriate range, so that the troubles described in the “BACKGROUND OF THE INVENTION” may be caused or the effect of the rust inhibitors cannot be fully achieved.